1. Field of the Invention
The present invention relates to non-flatting and slip-enhancing coating additives, to coating compositions containing the additives, and to a process for coating a substrate using the coating compositions.
2. Description of Related Art
Slip is the relative movement between two objects that are in contact with each other. If an object is moved along a surface, there is a resistance acting in a direction opposite the movement. The resisting force is also called frictional force, friction resulting from the unevenness of the two surfaces in contact.
Coating systems, including powder coatings are susceptible to damage when in contact with solid objects. Paint films, for instance, can be easily damaged during drying, when they have not yet cured. In the case of baking enamels, damage can occur when the films have not yet cooled. In addition, scratches, which are discontinuities on surfaces, can cause attachment of air-borne contaminants and corrosive agents leading eventually to corrosion problems. A common problem with many powder coatings is that they are subject to mar or scratching.
Slip additives provide some degree of protection against such damages. For example, in automotive coating applications, they protect the not fully hardened freshly applied enamels, and in can coatings, they protect the coating during filling and distribution.
The concepts of slip and mar (or scratch) resistance are very closely interrelated in that the same additive may perform both functions--impart slip and provide scratch resistance; in addition the principle underlying both is the same. A hard object touching a paint film containing an additive can be deflected due to the surface lubricity of the additive. This results in a greater apparent film hardness or scratch resistance.
A slip additive should generally fulfill several requirements. The additive should migrate to the film surface during the drying process, forming a film which not only adheres to the coating, but also favorably influences its flow and leveling properties. In addition, this film should have lubricating qualities, expressed as the ability to carry a load in a very thin layer.
Compounds based on silicone chemistry have been used as mar and slip additives for about 30 years. Examples of silicone-based slip additives include polydimethylsiloxanes, polyoxyalkylenesiloxane (polysiloxane/polyether) copolymers, and polyoxyalkylene methylalkylsiloxane copolymers.
The problem with many slip additives is that they reduce the gloss of the cured coating. The present invention provides enhanced slip (reduction of coefficient of friction) to cured powder coatings useful in rendering the cured coatings more resistant to mar or scratch without an appreciable loss in gloss.
A detailed description describing the concept of slip and mar and the developments of additives to improve scratch resistance and impart slip can be found in an article by F. Fink, et al., entitled "Development of New Additives to Improve Scratch Resistance and Impart Slip To Solvent-Based Coating Systems" in Journal of Coatings Technology, Vol. 62, No. 791, December 1990.
U.S. Pat. No. 4,898,779, Yoshimura et al, Feb. 6, 1990, discloses powders of tetrafluoroethylene copolymer having a non-spherical particle shape, a frictional packing ratio of 20 to 100% and an average particle size of 5 to 500 .mu.m. The powders are useful for powder coatings, particularly roto-moulding, roto-lining and electrostatic coating.
U.S. Pat. No. 5,346,727, Simkin, Sep. 13, 1994, discloses a process of coating a substrate such as aluminum by the steps of (a) applying thereon a powder coating composition prepared by (i) blending a mix containing a poly(vinylidene fluoride) resin, a thermoplastic resin, a pigment, and a vinylidene fluoride/tetrafluoroethylene/hexafluoropropylene terpolymer, (ii) pelletizing the blend from (i), and (iii) cryogenically grinding the pellets from(ii); (b) heating the powder coating composition above its melt temperature; and (c) cooling the coated substrate.
U.S. Pat. No. 5,405,912, Simkin, Apr. 11, 1995, discloses the use of vinylidene fluoride/tetrafluoroethylene/hexafluoropropylene terpolymer in polyvinylidene fluoride based powder coating resins to enhance the aesthetics of coatings made therefrom.
Japanese Patent 61-21977B, Tada et al., May 29, 1986, discloses a powdered smoothing agent for powder paints prepared by heating and fusing a mixture of an amide wax and an acrylic polymer. The amide wax can be made from a carboxylic acid, such as stearic acid and an amine, such as diethanol amine.
Japanese Patent Publication No. 63-44023, Suzuki et al., Sep. 2, 1988 discloses a method of forming a coated layer of low frictional resistance on the surface of an article such as a slip joint of a safety belt connected to a car seat, said method comprising immersing the article to be coated in a fluidized bed containing 100 parts weight of polyamide powder and 0.01-5 parts by weight of PTFE powder, and thereby applying the mixed powders as a low-friction coating on the surface of the particle.